M leicas have all been a vastly better investment than the R Leicas. An R4 sold new for nearly $2,000 and can now be had for $200 or so.
Leica M bodies hold value very well, relatively speaking. Certainly far better than anything else around, especially including digital cameras which have a rapid obscolescent technology depreciation factor.
Spend $1,000 on an M6 now, use it for five years, sell it for $1,000 if you take good care of it. I'd like to see any other camera on the market today do that.